Series strings of energy storage cells are extensively used as batteries in many applications, such as, as examples only, laptops, standby power supplies, electric vehicles. Imbalances in the charges of these cells tend to occur and grow over time, as the string is charged and discharged or even when the string is left charged but unused. This reduces the efficiency of the charging and discharging process, and also limits the life and capacity of the battery. It is therefore important to ensure the uniformity of charge for all cells in a battery or capacitor string. This is called balancing the cells.
When electrical capacitor cells connected in series are charged, each cell receives the same amount of charge. According to the capacity or state of health of the cell, the voltage across each of the cells may deviate from the expected or average voltage. Hence, as the energy E stored in a cell is given by E=CU2/2, whereby C is the capacitance of the cell, and U is the voltage across the cell, the voltage U may be different for each cell, depending on the capacity or state of health of the cell. The energy stored in a cell should be as high as possible for any cell; however, the voltage across a cell should not be higher than the maximum voltage allowed for that cell. Hence, as soon as the maximum voltage for one cell is reached, there must be refrained from further charging the string, although other cells are not necessarily loaded up to their maximum voltage yet. This results in a decreased performance of the complete string of series connected cells.
Prior art attempts to balance cells have included providing a constant shunt resistor over all individual cells (passive balancing) or a means for measuring the voltage of each cell, and then switching a resistor across those with a higher charge in order to discharge them to the level of the cell with the lowest charge (active balancing). An example thereof is described in US-2002/195994. However, as most cells perform substantially identical between pre-determined boundaries, and the cells which deviate most often have a lower than expected or desired charge, this causes a wasteful loss of energy, because lots of cells need to be discharged. Moreover, this system produces heat which is undesirable in a battery container.
Alternative arrangements rely on charge transfer between cells (dynamic balancing). An example thereof is described in EP-1283580, where each energy storage device is assigned a circuit balancing unit which circuit balancing units are connected such that they draw power from their assigned energy storage device if voltage balancing is performed, and forward it to the series connection of energy storage devices. Balancing is achieved by means of an inverter and transformer for each cell for common mode rejection. Hence, this arrangement requires isolating windings, and expensive and bulky components.
DE-102007045836 discloses a charging device having an electric accumulator formed by a plurality of series-connected electric accumulator cells. The charging device is capable of charging individual ones of a plurality of electric accumulator cells. However, it is a disadvantage of this charging device that only charging or not charging is possible. In order to be able to charge individual electric accumulator cells while others are not charged, DE-102007045836 describes that an AC generator is needed for each cell. This is a further disadvantage.
It is desirable to increase the voltage of individual cells with a low voltage and/or to decrease the voltage of cells with a high voltage, as charging is limited by the highest voltage cell and discharging is limited by the lowest, hence battery capacity is determined by the two extremes.